We plan to construct E. coli strains bearing plasmids which carry various cys genes from S. typhimurium. A strain carrying a cysB plasmid will be used as an enriched source of the cysB regulatory gene product and will enable us to more readily purify this protein. Using plasmid DNA carrying other cys genes, the interactions of the cysB regulatory protein with such DNA and with O-acetylserine will be studied. Additional studies will be carried out on the role of S-adenosylhomocysteine in the pathogenesis of the immune defect in adenosine deaminase deficient patients. This will require that we develop a more sensitive assay for S-adenosylhomocysteine than is currently available. We hope also to obtain samples of lymphocytes from adenosine deaminase deficient patients and analyze them for S-adenosylhomocysteine and for the state of methylation of their nucleic acids.